The present invention relates to apparatus and methods for ultrasonically measuring the depth of precipitation accumulating on the surface of a pathway, such as a highway, a bridge, an airport runway, or a building floor. More specifically, the present invention relates to apparatus and methods for measuring the depth of precipitation on a surface of a sensor embedded in the pathway by emitting pulses of ultrasonic energy which are reflected as first and second reflections from the surface of the sensor and from the upper surface of the accumulation of precipitation respectively.
Easily ascertainable and accurate knowledge of the depth of water accumulating on the surfaces of airport runways, for example, is very useful and can be indispensable. When an aircraft lands on a runway covered with water, a condition called hydroplaning can occur and cause a dangerous skid. The minimum depth of water at which hydroplaning first occurs is inversely related to the landing speed of the aircraft, generally speaking.
During a rainstorm hundreds of lives can depend on deciding which runways are safe to use and which are not, because aircraft certainly should not be allowed to land on runways which are unsafe for them. The safety problem is complicated, however, because the aircraft should not be unnecessarily detained in the air in bad weather either. In other words, attention to safety by unnecessarily closing runways keeps aircraft in the air longer in bad weather, can congest the available airspace, and unnecessarily jeopardizes the level of safety in the air. Consequently, only those runways should be closed which are actually hazardous to the particular aircraft requesting clearance to land.
Whether a hydroplaning hazard exists depends on variables such as water depth, landing speed, wind direction and speed, and the weight of the aircraft. Although water depth is an important variable in determining whether hydroplaning is likely, accurate water depth information for airport runways has not generally been available.
The airport operations manager should be able to discern, at a glance, runways or portions thereof on which the danger of hydroplaning actually exists for particular aircraft in order to select other runways for landing. Also, it should be possible to determine when a given runway is hazardous for all aircraft so that it can be closed. Similarly, warning information is needed as soon as a wet runway is becoming icy.
Coassigned U.S. Pat. No. 4,986,110, describes a water depth measuring device which utilizes capacitance and conductance properties of water in detecting its presence. Also, circuitry is described therein for producing indications of depth as being less than 0.05 inch, between 0.05 inch and 0.10 inch, or greater than 0.10 inch.
In coassigned U.S. Pat. No. 4,135,151, precipitation on the sensor is identified as water, slush, or ice by a capacitance and conductance approach. In coassigned U.S. Pat. No. 4,281,286 at least two different frequencies are used in a capacitance and conductance approach, depending on whether impurities are or are not present in the precipitation.
Accurate measurement of values in a substantially continuous range of precipitation depth has not been possible in that the capacitance and conductivity of the precipitation vary considerably with impurity type and concentration, and with temperature. Accurate measurements of very small depths on the order of 1/32 inch are also needed.
Because of practical problems of surface turbulence and roughness due to wind, snow, and pelting precipitation, the problem of depth measurement is further complicated. One reference "Industrial Applications of Ultrasound--A Review: II. Measurements, Tests, and Process Control Using Low-Intensity Ultrasound," by L. C. Lynnworth, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-22, No. 2, March, 1975 at page 89, only mentions ultrasonic measurements of thickness in the industrial environment, and only recognizes rough surface and poorly defined thickness conditions as being special problems.
Measurement of precipitation depth is frought with difficulties because it must be performed out-of-doors under windy and rainy or snowy conditions when the measurements are most needed. Even defining precipitation depth under such conditions is troublesome, and any raw data even if accurate can be expected to be only obtainable sporadically and erratically. Practical application of depth measuring equipment also demands that it require but little and infrequent adjustment and have a minimum of down-time for maintenance and calibration.